To make the salad. Preheat the grill to medium-high. Place the peppers skin-side up on a baking tray, and grill for 15 minutes until charred. Place in a plastic bag. Cool, peel and slice into strips. Place in a large bowl with the red onion, olives and basil. Season.
To make the salad dressing: Whisk together the olive oil, lemon juice, garlic and mustard. Season and taste then mix it through the salad. Set aside while you cook the mackerel.
To cook the mackerel: Preheat the grill to high. Line a grill pan or baking tray with lightly oiled tin foil then add the fish, skin side down. Mix the olive oil, paprika and cumin together, and brush half of this mixture over the flesh side of the mackerel. Reserve the rest to brush over the bread. Season the mackerel then place under the hot grill and cook for approximately 6 minutes.
Brush the reserved oil over the bread and grill on each side until golden.
To serve: Serve the mackerel with the salad and the bread on the side or take the skin of the mackerel and serve it and the salad on the bread.